In fuel-cell-supported transportation systems, so-called chemical reformers are used for extracting the required hydrogen from hydrocarbon-containing fuels.
All the substances needed by the reformer for the course of reaction, such as air, water, and fuel, are ideally supplied to the reformer in the gaseous state. However, since the fuels, such as methanol or gasoline, and water are preferably present onboard the transportation system in liquid form, they must be heated shortly before being supplied to the reformer, in order to vaporize them. This requires a pre-evaporator capable of providing adequate quantities of gaseous fuel and water vapor, the waste heat of the reformer mostly being used for vaporization.
Since the hydrogen is normally consumed immediately, chemical reformers must be capable of adjusting the production of hydrogen to the demand without delay, e.g. in response to load changes or during start phases. Especially in the cold start phase, additional measures must be taken, since the reformer does not provide any waste heat. Conventional evaporators are not capable of generating adequate quantities of gaseous reactants without delay.
So-called catalytic burners provide the temperature required for the chemical reaction, in which, e.g. the fuel is reformed to form, among other things, hydrogen. Catalytic burners are components that have surfaces coated with a catalyst. In these catalytic burners, the fuel/air mixture is converted into heat and exhaust gases, the generated heat being conducted to the suitable components such as the chemical reformer or an evaporator via, for example, the (lateral) surfaces and/or via the warm exhaust-gas stream.
The conversion of fuel into heat is highly dependent on the size of the fuel droplets striking the catalytic layer. The smaller the size of the droplets and the more uniformly the catalytic layer is wetted with the fuel droplets, the more completely the fuel is converted into heat and the higher the efficiency. In this way, the fuel is also converted more quickly and pollutant emissions are reduced. Fuel droplets that are too large in size result in the coating of the catalytic layer and hence, in a slow conversion rate. This leads to, e.g. poor efficiency, especially in the cold start phase.
It is therefore practical to introduce the fuel into the reformer/catalytic burner in a finely dispersed form, with the aid of an atomization device, in which case, provided that there is a sufficient supply of heat, the vaporization process is improved by the high surface area of the finely dispersed fuel.
Devices for metering fuels into reformers are known, for example, from U.S. Pat. No. 3,971,847. According to this document, metering devices located relatively far away from the reformer are used to meter the fuel via long supply lines and a simple nozzle into a temperature-adjusted material stream. In the process, the fuel first strikes baffle plates positioned downstream from the nozzle outlet orifice, which are designed to swirl and disperse the fuel before arriving, via a relatively long vaporization section (path) necessary for the vaporization process, at the reaction region of the reformer. The long supply line allows the metering device to be insulated from thermal influences of the reformer.
A particularly disadvantageous feature in the devices known from the above-mentioned document is the fact that, due to the simple construction of the nozzle and the positioning of the baffle plates, a targeted metering of fuel, for example into regions of the reformer that have a large supply of heat, is possible only to an insufficient degree. This leads to the need for a relatively large space due to the necessity of a long and voluminous vaporization section.
Furthermore, problems arise in cold start operation, since long and voluminous vaporization sections are slow to heat up and also give off a relatively large amount of heat unused. The set-ups of nozzle and baffle plates described in U.S. Pat. No. 3,971,847 particularly do not allow the interior surface of a hollow cylinder to be uniformly wetted with fuel and, in so doing, do not allow certain surfaces of the hollow cylinder to be excluded from being wetted with fuel, or the quantity of the metered fuel to be adjusted to the distribution of the supply of heat in the metering space. Even the shape of the fuel cloud resulting from the metering process can be influenced only to an insufficient degree.
A further disadvantage is that the shape of the fuel cloud or the distribution of the dosed-in fuel may not be adequately controlled by adjusting the baffle plates.